In a conventional transmitter system, a passive matching network is inserted between the output of the Power Amplifier (PA) and the antenna to match the output impedance of the PA (typically 50 ohms) to that of the antenna, as shown in FIG. 1. Passive matching networks typically include one or more capacitors and one or more inductors arranged in well-known configurations. These passive networks receive power only from their source (typically the PA) and do not need any external source of power. These passive networks comply with Foster's Reactance Theorem. Passive Foster networks work fairly well in matching the impedance of the PA to the impedance the antenna at a single frequency. But they are not perfect, even at a single frequency, since inductors and capacitors in the real world are non-ideal, that is, they have resistance in addition to reactance. Moreover, most practical applications require a transmitter to operate over a bandwidth and especially when physically small size antennas are utilized, it is often not possible to achieve an acceptable or desirable impedance match over an acceptable or desirable bandwidth using Foster (passive) networks. Antennas of small physical size are frequently used in hand-held applications such as cell phones, smart phones, lap top or smaller computers, and so on. For additional background information, see Stephen E. Sussman-Fort, Ron M. Rudish, “Non-Foster Impedance Matching for Transmit Applications,” 2006 IEEE International Workshop on Antenna Technology Small Antennas and Novel Metamaterials, pp. 53-56, Mar. 6-8, 2006 and Stephen E. Sussman-Fort, Ron M. Rudish, “Increasing Efficiency or Bandwidth of Electrically-Small Transmit Antennas by Impedance Matching with Non-Foster Circuits,” PIERS 2006, Mar. 26-29, 2006.
Better impedance matching can be achieved using Non-Foster (or active) networks. Non-Foster (or active) networks require a source of power (typically DC) in addition to that provided by merely being coupled to the output of the PA. These networks may utilize a Negative Impedance Convertor (NIC). So in an actively matched transmitter system, a NIC can be inserted between the PA and the antenna to cancel the reactance of the antenna, while another resistance matching network provides the resistance match to the antenna, as shown in FIG. 2. A number of different designs for NICs are known per se in the prior art. See, for example, Stephen E. Sussman-Fort, “Gyrator-Based Biquad Filters and Negative Impedance Converters for Microwaves,” International Journal of RF and Microwave Computer-Aided Engineering, Vol. 8, No. 2, pp. 86-101, 1998, the disclosure of which is hereby incorporated herein by reference.
A disadvantage of the configuration of FIG. 2 is that the RF voltage swing across the NIC is scaled up by approximately Xant/Rant. The DC bias voltage of the NIC needs to be scaled up also to support this swing, hence causing higher power dissipation. This directly impacts the efficiency of the transmitter.
The present state-of-the-art of power amplifiers for antennas with non-Foster active matching use a NIC to invert the phase of an inductive or capacitive element to provide a broadband cancellation of the reactance of an electrically small antenna. This improves the efficiency and bandwidth of the PA by presenting it a resistive load, but comes at a cost of high power dissipation in the NIC. Despite the improved match, the combined power dissipation of the NIC and PA in a typical prior art application exceeds that of just having the PA drive an unmatched antenna, a consideration which has thus far limited the practicality of using NICs in real world applications.
Alternative techniques include using more complex matching structures and antenna designs, but these can only approach the well-established Chu limit on bandwidth for passive matching to a reactive antenna as a function of the antenna size. The circuits disclosed herein break (exceed) the Chu limit on bandwidth by using non-Foster impedances for matching, while also circumventing the limitations of current non-Foster matching techniques.
Impedance matching of the PA to antenna has been conventionally treated as an interface problem where the solution was provided either by passive impedance matching network or by an active matching circuit. The use of a NIC as the active matching circuit has been proposed before but did not receive a lot of attention because of the difficulty in making a stable and efficient NIC, especially in a transmit application. This disclosure addresses the efficiency problem. For stability issues the reader is directed to Stephen E. Sussman-Fort, “Matching Network Design Using Non-Foster Impedances,” International Journal of RF and Microwave Computer-Aided Engineering, Vol. 16, Issue 2, pp. 135-142, February 2006.